A carbon nanotube (hereinafter may be referred to as “CNT”) is a carbon structure such that a carbon sheet, which is constituted by carbon atoms arranged two-dimensionally in the form of hexagons, is closed to form a cylindrical shape. The CNT may consist of multiple walls or of a single wall. Because of their mechanical strength, optical characteristics, electrical characteristics, thermal characteristics and molecular adsorption capacity etc., both of such CNTs show promise as functional materials such as an electronic device material, an optical element material and an electrically conducting material. Out of the CNTs, in particular a single-wall CNT has been attracting attention as a material for a nano-electronic device, a nano-optical element or an energy storage device, because of its excellent characteristics such as excellent electrical characteristics (very high current density), excellent thermal characteristics (heat conductivity equivalent to diamonds), excellent optical characteristics (light emitting property in a wavelength range for optical communication), excellent hydrogen storage property and an excellent metal catalyst supporting property etc. as well as its characteristics of both semiconductor and metal.
In order to make good use of CNTs for such purposes, it is desirable that a plurality of CNTs be each aligned in a specific direction and gathered to form an aggregate in the form of a bundle, a film or a block, and that such a CNT aggregate deliver their functions such as electrical, electronic and optical functions. A CNT is a material that has a one-dimensional structure with very high aspect ratio, and thus its function shows high directivity. Therefore, arranging CNTs constituting a CNT aggregate (structure) such that they are each aligned in a specific direction makes it possible to cause functions thereof to have the same directivity. This makes it possible to obtain a high-performance CNT aggregate.
That is, an aligned CNT aggregate in which the CNTs are each aligned in a specific direction shows high directivity of transmission characteristics, i.e., its transmission characteristic in the specific direction is high, as compared to a CNT aggregate in which CNTs are each randomly oriented, i.e., a non-aligned CNT. Because of such a high directivity, the CNT aggregate shows better electrical characteristics (e.g., higher electric conductivity), better mechanical characteristics (e.g., higher strength) and better thermal characteristics (e.g., higher heat conductivity). Further, such a characteristic of the CNT aggregate that differs between in the specific direction and in other directions, i.e., anisotropy of the CNT aggregate, is effective when for example heat is desired to be diffused or discharged selectively in a desired direction, and thus such a CNT aggregate is suitable for use as a heat conductive material etc. Further, the CNT aggregate is desirably larger in size such as height and length. It is expected that newly developing such an aligned CNT aggregate will dramatically increase the field of application of CNTs.
Meanwhile, there has been known chemical vapor deposition (hereinafter may be referred to as “CVD”), which is one of the methods of producing CNTs (refer to Patent Literatures 1 and 2 etc.). The CVD is characterized by bringing a carbon compound into contact with fine metal particles serving as a catalyst in a high temperature environment of approximately 500° C. to 1000° C. Since the CVD enables production of CNT under the conditions where the kind and arrangement of catalyst or the kind and reaction conditions of a carbon compound etc. are variously changed, the CVD has been attracting attention as being suitable for mass production of CNTs. Further, the CVD is advantageous in that (i) it is possible to produce both single-wall carbon nanotubes (SWCNT) and multiwall carbon nanotubes (MWCNT) and (ii) since a substrate on which a catalyst is supported is used, it is possible to produce a large number of CNTs aligned in a direction perpendicular to a surface of the substrate.
For example, Patent Literature 2 discloses a method of allowing CNTs to form on a silicon substrate by chemical vapor deposition in the presence of an oxidizer, which method is for industrial production of CNTs. This method aims at improving purity of CNTs, increasing specific surface area of CNTs and improving degree of alignment of CNTs.